US President-elect Donald Trump has said it was a “great honour” to meet President Barack Obama for transition talks at the White House.
Mr Obama said he was “encouraged” by their “excellent” and “wide-ranging” conversation.
Mr Trump has questioned Mr Obama’s US citizenship and vowed to dismantle his legacy in the past.
During the campaign, Mr Obama called Mr Trump “uniquely unqualified” but later said he was “rooting” for him.
On Wednesday, thousands took to the streets of major US cities denouncing Mr Trump after his shock defeat of Hillary Clinton.
White House spokesman Josh Earnest has insisted Mr Obama is sincere about ensuring a smooth handover when he meets Mr Trump, though he added: “I’m not saying it’s going to be an easy meeting.”
Mr Trump flew from New York on his private jet and landed at Reagan National
Airport, just outside the nation’s capital.
The president-elect broke from protocol and barred journalists from travelling with him to cover his first meeting with Mr Obama.
The two men are expected to appear together for the cameras in the Oval Office after a behind-closed-doors meeting.
The president-elect is being accompanied by his wife, Melania, who will have a meeting with First Lady Michelle Obama.
Mr Obama, who congratulated his successor in a phone call in the early hours of Wednesday, said it was “no secret” that he and Mr Trump had pretty significant differences.
But the Democratic president – who had campaigned against Mr Trump – urged all Americans to accept the result of the presidential election.
“We are now all rooting for his success in uniting and leading the country,” he said.
The defeated Mrs Clinton also told supporters Mr Trump had to be given a “chance to lead”.
Despite the pleas for unity, protesters gathered across the country on Wednesday. Many chanted: “Not my president.”
In New York, thousands marched on Trump Tower. Sixty-five people were arrested.
During demonstrations in Oakland, California, some shop windows were smashed and missiles hurled at riot police, who responded with tear gas.
A mass anti-Trump rally shut down the key 101 freeway in Los Angeles.
In Chicago, crowds blocked the entrance to Trump Tower, chanting: “No Trump, No KKK, No Fascists USA”.
Protests also took place in Washington DC, Philadelphia, Boston, Seattle, San Francisco and Portland, Oregon.
Mr Obama and Mr Trump have a history of mutual hostility.
Mr Trump led the charge in challenging the legitimacy of Mr Obama’s presidency through the “birther” movement, which falsely claimed the Hawaii-
born commander-in-chief was actually born outside the US.
The businessman also called Mr Obama “the worst president in the history of the United States”.
For his part, the president famously skewered Mr Trump in person at the 2011 White House Correspondents’ Association Dinner, which some have suggested may have spurred the New York billionaire to seek revenge.
With the Republicans now holding a majority in both chambers of the US Congress, Mr Trump can more easily target key Obama initiatives like such as his healthcare reforms.
Vice-President-elect Mike Pence, a favourite among social conservatives, is also meeting behind closed doors on Thursday with Vice-President Joe Biden, one of the Democratic party’s most popular figures.
Mr Trump is already setting up bilaterals ahead of his January inauguration as the nation’s 45th US president.
The office of British Prime Minister Theresa May said on Thursday that the president-elect had invited her in a phone call to visit him “as soon as possible”.
Ms May’s office said in a statement that Mr Trump had praised the trans-Atlantic “special relationship” while discussing his “close and personal connections” with the UK.
Mr Trump has family and business ties to Scotland.
The president-elect’s transition team for the 10-week period until inauguration will be led by Chris Christie, Governor of New Jersey.
Mr Trump, who has never held elected office, has said his immediate priorities will be restoring the country’s infrastructure and doubling its economic growth.
As president-elect, Mr Trump is entitled to get the same daily intelligence briefing as President Obama, which includes information on covert US operations and other data gathered by America’s 17 intelligence agencies.
Mr Trump’s team is understood to be focused on quickly filling key national security posts.
It is not yet clear who will sit his cabinet or fill senior posts in his administration, such as chief of staff, but several figures in his inner circle have been mentioned.
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